1. Field of the Art
This invention relates to a liquid crystal display device displaying an image employing a light transmissivity of liquid crystal, and more particularly to a residual image eliminating apparatus and method that is adaptive for eliminating a residual image emerging on a screen due to a residual electric charge accumulated in a picture element (or pixel) cell after a power source was turned off.
2. Description of the Related Art
Recently, there has been an accelerated development of a flat panel display device of an active matrix driving system, for example, a liquid crystal display device using thin film transistors (TFTs) as switching devices. Since such a liquid crystal display apparatus can have a smaller dimension in comparison to the existing cathode ray tube (or brown tube), it has been commercially available for a display device of a portable television, a lap-top personal computer, and so on.
Referring to FIG. 1, there is shown a pixel cell of a liquid crystal display panel that includes a TFT 10 having a gate connected to a gate line 11 and a source connected to a data line 13, and a parallel connection of a liquid crystal cell 12 and a support capacitor 14 between a drain of the TFT 10 and a common voltage source Vcom. The TFT 10 is turned on with a voltage higher than a threshold voltage applied to the gate thereof upon displaying of a picture, thereby connecting the data line 13 to the liquid crystal cell 12 and the support capacitor 14. The liquid crystal cell 12 and the support capacitor 14 accumulate a voltage of an image signal Vd from the data line 13 when the TFT 10 is turned on, and maintains the accumulated voltage until the TFT 10 is turned on again. Upon line inversion driving, the polarity of the common voltage Vcom is inverted depending on the gate line 11, thereby supplying the adjacent gate lines with a common voltage Vcom having the contrary polarity with respect to each other.
When a power source of the liquid crystal display panel is turned on, a gate low voltage Vg1 having a voltage level less than the gate threshold voltage Vth is supplied to gate lines 11, excluding the gate line coupled with the image signal Vd. This gate low voltage Vg1 is set to have a value lower than the minimum value of the image signal Vd. On the other hand, when a power source of the liquid crystal display panel is turned off, the gate low voltage Vg1, the image signal Vd and the common voltage Vcom are converged into a specific level (i.e., a voltage level corresponding to a ground voltage supplied during operation of the liquid crystal display panel, hereinafter referred to as “ground level” GND). At this time, the gate low voltage Vg1 changes as shown in FIG. 2. Typically, the liquid crystal display device includes a residual image eliminating apparatus for eliminating a residual image by converging the gate low voltage Vg1 to the ground level GND after a power source of the liquid crystal display panel was turned off.
As shown in FIG. 3, the residual image eliminating apparatus includes a zener diode ZD for maintaining the gate low voltage Vg1 to be supplied to the gate line 11 at a predetermined level, and a transistor Q1 for switching a current path for converging the gate low voltage Vg1 into the ground level GND when a power source of the liquid crystal display panel was turned off. Also, the residual image eliminating apparatus has a capacitor C1 connected between a positive voltage line PVL and the base of the transistor Q1. The zener diode ZD is commonly connected to the gate low voltage line VGLL and the emitter of the transistor Q1 to always lower a negative voltage VEE from a negative voltage line NVL into its breakdown voltage, and supplies the lowered voltage to the gate low voltage line VGLL. For example, if the negative voltage VEE is −5V and the breakdown voltage of the zener diode ZD is 1V, then the gate low voltage Vg1 becomes −6V. The transistor Q1 is a PNP-type transistor which receives a voltage VDD having a positive level (e.g., 5V or 3.3V) from the positive voltage line PVL at the base thereof through the capacitor C1 when a power source of the liquid crystal display panel is turned on. At this time, since almost an infinite value of resistance exists between the emitter and the collector of the transistor Q1, the gate low voltage Vg1 on the connection node between the zener diode ZD and the transistor Q1 is not bypassed into the ground voltage GND, but it is supplied to the gate low voltage line VGLL. Meanwhile, the capacitor C1 charges the positive voltage VDD from the positive voltage line PVL.
When a power source of the liquid crystal panel is turned off, the ground voltage GND is developed on each of the negative voltage line NVL and the positive voltage line PVL. At the same time, the capacitor C1 applies a negative polarity voltage—VDD to the base of the transistor Q1 by the charged electric charges thereof. Then, the transistor Q1 is turned on by converging the positive voltage VDD into the ground level GND, thereby connecting its emitter to the collector. The gate low voltage Vg1 is converged into the ground level GND by turning on the transistor Q1. The zener diode ZD is turned off by converging the negative voltage VEE [and the gate low voltage Vg1] into the ground level GND.
On the other hand, upon line inversion driving, the common voltage Vcom having an alternating current shape as shown in FIG. 4 is supplied to the liquid crystal cell 12 and the support capacitor 14. During line inversion driving, the gate low voltage Vg1 is supplied to the gate line 11 in a shape of alternating current synchronized with the common voltage Vcom by means of an alternating current source AC and a coupling capacitor Cc. When a power source of the liquid crystal display panel is turned off, the common voltage Vcom is converged into the ground level GND. At this time, A side pixels charged with a negative polarity level with respect to the ground level GND and B side pixels charged with a positive polarity level with respect to the ground level GND exist in the liquid crystal display panel. If a power source of the liquid crystal display panel is turned off, then a channel of the TFT is turned on because the image signal Vd, the gate low voltage Vg1 and the common voltage Vcom are charged into the ground level GND and a negative polarity voltage with respect to the ground level GND is charged in the A side pixel. Accordingly, the voltage charged in the A side pixel is converged into the ground level GND. In other words, when a negative(−) voltage is charged into the liquid crystal cell 12 based on the ground level GND, a voltage applied to the gate of the TFT 10 becomes higher than a pixel charge voltage Vp. As a result, electric charges charged in the liquid crystal cell 12 are bypassed into the data line 13, so that a residual image does not emerge at the corresponding lines.
Otherwise, since a channel of the TFT connected to the B side pixel charged with a positive(+) voltage with respect to the ground level GND is turned off, the pixel voltage Vp is converged into the ground level GND slowly. In other words, in the case of the liquid crystal cell 12 charged with a positive (+) voltage based on the ground level GND before the power source is turned off, a voltage applied to the gate of the TFT 10 becomes lower than the pixel charge voltage Vp. Accordingly, even though a power of the liquid crystal display panel is turned off, a residual image emerges on a screen (i.e., a liquid crystal display panel). Further, in the case of being driven in the line inversion system, a residual image appears at odd-numbered gate lines 11 or even-numbered gate lines 11. It takes a considerable time (i.e., more than about one minute) to extinguish such a residual image.